The theoretical efficiency of a solar cell is high, but the actual power generation efficiency is not the case. There are many factors affecting the cell efficiency, such as series and parallel resistances of the solar cell itself, the shielding of sunlight by the solar cell electrodes, or the loss caused by not effectively capturing the reflected light, etc.
The recombination of electrons and holes inside solar cell is another important factor affecting the cell efficiency. The absorption of incident photons having greater energy than the band gap of a solar cell creates electron-hole pairs, and these carriers are separated by the action of the electric field existing at the p-n junction of the solar cell. The light-generated minority carriers (i.e. electrons or holes in the electron-hole pairs) reach the p-n junction, and sweep across the junction, wherein they become majority carriers. If the solar cell is short-circuited, the light-generated carriers flow through an external circuitry to complete a circuit. The power generation efficiency of solar cell is affected by the recombination rate of electrons and holes in the cell, resulting in depletion of charge carriers.
Standard silicon solar cell composes of a base silicon region, an emitter silicon region, p-n junction region, front electrical contact (front electrode) and rear electrical contact (rear or back electrode). In recent years, several type of high-efficiency crystalline silicon solar cells, such as heterojunction with intrinsic thin layer (HIT), passivated emitter and rear cell (PERC), and passivated emitter rear locally diffused cell (PERL) solar cells have been developed to improve the said recombination loss.
Dielectric passivation is a commonly used method to minimize the carriers recombination at the surface of a solar cell. The passivation materials include any suitable materials capable of holding either positive or negative charges. The application of charges with a reverse polarity with respect to the semiconductor emitter or base layer can create electric field that repels minority carriers from moving through the solar cell, thereby reducing the carriers' recombination. The two well-known surface passivation methods are chemical passivation and field-effect passivation.
The chemical passivation includes depositing a dielectric material on the surface of a n-type or p-type semiconductor with a thermal oxidation process, which is one of the high-temperature (e.g. 1000° C.) surface passivation techniques. The thermal oxidation process provides a good interface passivation quality through chemical bonding between the dielectric layer and the semiconductor, and results in defect density reduction at the interface of the semiconductor/dielectric layer, thereby lowering the recombination probability of the electrons and holes on a semiconductor surface. However, the high temperature condition may cause carriers lifetime degradation and dopants redistribution at the n+/p interface. The thermal oxidation process can be replaced by plasma-enhanced chemical vapor deposition (PECVD) method, in which the plasma is excited at a lower temperature of about 400° C., and some unstable defects of the solar cell are repaired with hydrogen.
The field-effect passivation approach adopts dielectric material to inhibit the recombination of minority carriers. The surface passivation property of a dielectric layer depends crucially on the fixed charges in the dielectric layer and the doping concentration of the semiconductor. For n-type semiconductor, silicon nitride having positive charges induces majority carriers (electrons) accumulation at the dielectric/semiconductor interface, causing the energy bands to bend downward. The minority carriers (holes) are shielded from the crystalline silicon solar cell surface.
Another type of field-effect passivation forms a heavily-doped p+ region as a barrier on the back surface of a p-type semiconductor, so as to prevent the minority carriers (electrons) from moving to the back contact. A back surface field (BSF) which consists of a higher doped region at the rear surface of a solar cell, is one kind of such passivation. The electric field formed at the interface between the high and low doped regions induces a barrier that repels minority carriers from first electrode. However, sometimes additional doping with acceptor-type dopant required to form the p+ region, demands extra manufacturing steps and higher processing costs.
From the view of structure, the dielectric passivation layer formed by both the chemical passivation and the field-effect passivation methods directly contacts the semiconductor photoelectric conversion layer of a solar cell.
In contrast to the above mentioned dielectric passivation, the word “passivation” has another meaning in term of solar cell reliability. Polymer encapsulant is one of the well known materials to provide an electrically insulating passivation layer with respect to an electrically conductive layer applied thereto, such as an electrode layer. This polymer encapsulant adheres to both the cathode and the anode of the solar cell simultaneously, to provide durable, long-lasting protection for solar cell against environmental hazards.
Presently, there are several types of polymer encapsulants available, including ethylene vinyl acetate (EVA), polyvinyl butyral (PVB), polydimethyl siloxane (PDMS), thermoplastic polyolefins (TPO), thermoplastic polyurethane (TPU), ethylene propylene diene monomer (EPDM) and ionomer. According to Kempe's study “Overview of scientific issues involved in selection of polymers for PV applications” presented at the 37th IEEE Photovoltaic Specialist Conference (2011); the volume resistivities of EVA, TPU and polyolefin are of the order of E14 Ohm/sq, while silicone, ionomer and EPDM are about 100 times more resistive than EVA. The resistivity of polymer encapsulant is relevant to electrical insulation.
It is worth mentioning that an ionomer is a charged copolymer containing both electrically neutral repeating units and a fraction of ionizable ionic group covalently bonded to the polymer backbone. In this type of polymer, the ionic association mainly contributes to the thermoplastic properties of the material. Below the melt temperature, this polymer aligns to form physical crosslinking due to ionic groups attraction. However at elevated temperature this ionic interaction disappears and the polymer chain will move around freely. This thermally reversible thermoplastic behavior allows the ionomer to have easy processibility at elevated temperature and high modulus at room temperature, thus making the ionomer suitable for solar cell encapsulant applications.